The present invention relates to a method and apparatus for coating both sides of a moving web, such as a paper web, moving at a rate in excess of 200 m/min, whereby the same or two different coating agents are applied simultaneously to each side of the web and the coating agent thus applied is smoothed out by means of two opposing blades each having a bevelled edge acting on the web.
Several different methods and apparatus operating with two opposing, flexible blades are already known for simultaneously coating both sides of a paper web. In one known proposal the paper web is passed from the top downwardly, a bath of coating compound being contained in the space formed between the opposing blades with their holders. These blades form an acute angle to the direction of movement of the web.
In other known embodiments the paper web is passed substantially vertically upwards, the smoothing effect being obtained by means of two opposing blades which in this case also form an acute angle to the direction of movement of the web. The embodiments mentioned above have the advantage that because of the forces of gravity prevailing the excess coating compound can be recirculated in the equipment. Due to the hydraulic forces occurring resulting from the moderately vigorous movement in the coating compound during the coating process, a force is obtained on the coating blades which counteracts the spring force of the blades themselves when they are pressed against the paper web.
Regulation of the final quantity of coating compound applied is therefore dependent on the pressure exerted by the blades upon each other. This pressure can be effected in various ways. At high speeds of the web and/or high viscosity of the coating compound, the increased hydraulic pressure on the blades must be compensated by an increased pressure on the blades so that the desired application quantity is obtained.
In known arrangements for coating using two opposing blades, the paper web does not have a movable support such as a rotating roller, as in the case of traditional blade coating. There is therefore a greater risk of defects in the coating or breakages in the paper web caused by particles in either the coating compound or the paper web catching between one or both coating blades and the moving paper web. Particles caught in this way may easily result in longitudinal lines in the coating layer (so-called streaks) so that the coated paper must be rejected or, if the particles are large or sharp the web may be perforated and a rupture may occur.
This drawback is inherent in conventional means for coating with two opposing blades and increases with increasing web speeds and/or high dryness contents or viscosities of the coating compound. It has been found, for instance, that when using traditional steel blades suitable for blade coating in traditional thicknesses (0.30 - 0.60 mm), it is impossible to achieve an acceptable result with respect to rejection and production at web speeds above 150 - 200 m/min.